IMX6 sabresd: mmcblk0 instead of mmcblk2 for SD3

I was provided with custom Linux 3.18 kernel. It is not BSP. I could not make it to boot until i changed mmcroot from mmcblk2p2 to mmcblk0p2. However, from user's guide "The SD3 slot uses mmcblk2 on i.MX 6". Is this because of non BSP kernel? Could not find any information to confirm this. I am sorry, just started embedded development.